1. Field of the Invention
The present invention relates to a method of manufacturing side rails for a combined oil ring, which is principally used in an internal combustion engine, and has a pair of side rails separated by a space expander which forces the pair of rails to scrape off surplus oil on an inner wall of a cylinder.
2. Description of the Related Art
With the recent trend toward higher loads in an internal combustion engine, a nitriding treatment has been applied to a side rail of a combined oil ring made of steel (see Japanese Patent Publication No. 61888/1982 or Japanese Patent Laid-Open No. 136771/1983 publication). This treatment of the side rails will cause an abnormal abrasion phenomenon in an oil ring groove of a piston made of aluminum alloy in some engines which use lead-free gasoline. The cause of the abnormal abrasion of the oil ring groove is considered to be due to the following:
The hardness of a nitrided layer is too high as compared with the hardness of the aluminum alloy.
A combined oil ring is received in an oil ring groove of a piston, and a side rail will rotate about an axis while an end portion of the rail vibrates up and down.
When a relative rotation between the side rail and the oil ring groove is impaired for some reason, the wall surface of the oil ring groove sometimes becomes abnormally worn by a nitrided hard and sharp edge of the end portion or surface at a gap of the side rail.
The following measures have been taken with regard to a side rail in consideration of the above points:
(1) A chamfered portion is provided in the inner periphery of the side rail to reduce the friction during rotation within the oil ring groove (see Japanese Utility Model Laid-Open No. 34343/1988 publication). PA1 (2) Smooth chamfers are applied to corner portions between an end surface of a gap of the rail and the upper and lower surfaces of the side rail (see Japanese Utility Model Laid-Open No. 178274/1989 publication). PA1 (3) The end portions at the gap of the side rail are prevented from being nitrided (see Japanese Utility Model Laid-Open No. 72863/1990 publication).
However, when the nitriding treatment is applied to the upper and lower surfaces of the side rail, the above-described measures of (1) or (2) may sometimes be insufficient.
On the other hand, the nitriding treatment which is applied to a piston ring is carried out by gas nitriding method or salt-bath nitriding method. According to these methods, not only the outer peripheral surface but also the upper and lower surfaces and the inner peripheral surface become nitrided. Thus, all surfaces of the rail become nitrided as just mentioned. Therefore, when a thick nitriding is applied to a thin ring or rail, the brittleness increases with nitriding, and the breaking strength becomes materially lowered. This structure will not respond to the requirement of the thinner ring with a high abrasion resistance. Either unnecessary nitrided portion should be removed by machining after a nitriding treatment or an anti-nitriding treatment should be applied in order that the nitrided layers are not provided on the upper and lower surfaces of the side rail. These steps or treatments involve considerable disadvantages in terms of technique as well as cost.
In the anti-nitriding treatment described in the above-mentioned item (3), a plating of nickel, copper, etc., is applied to the end portion of the rail, which plating is a disadvantageously cumbersome step or process.